This invention relates to a method for attaching an integrated circuit component to a printed circuit board by solder bump interconnections. More particularly, this invention relates to a method for forming such interconnections between a circuit trace on the board that carries a continuous solder plate formed of a relatively low melting solder alloy and a solder bump affixed to the component formed of a relatively high melting solder alloy. In one aspect of this invention, the component includes spacer solder bumps in addition to the bumps required for the interconnections, which spacer bumps maintain a spacing between the component and the board to prevent collapse during solder reflow operations to fuse the alloys to complete the interconnection.
In the manufacture of a microelectronic package, it is known to mount an integrated circuit component to a printed circuit board by a plurality of solder bump interconnections that not only physically attach the component, but also electrically connect an electrical circuit of the component to an electrical circuit on the board for conducting electrical signals to and from the component for processing. For this purpose, the board comprises a circuit trace disposed on a dielectric substrate and formed of a solder-wettable metal, typically copper. The trace features terminals, each having a terminal pad adapted for bonding the interconnection, and a runner extending from the pad for electrical communication with remote elements of the circuit. The component, which may be, for example, an integrated circuit die, has a plurality of solderable bond pads arranged in a pattern superposable onto the terminal pads.
To form the interconnection, a solder bump is affixed to each bond pad of the component. This is commonly accomplished by placing a preformed microsphere of solder alloy onto each pad and briefly heating to reflow the alloy to bond the solder to the pad and form the bump. The component is then assembled with the board such that the bumps rest against the terminal pads on the board. The assembly is briefly heated to again reflow the solder alloy and thereby bond the bump to the terminal pad on the board to complete the interconnection.
During reflow onto the board, the molten solder alloy wets the copper trace surface to provide the intimate contact that is essential for forming a strong solder bond. This is accompanied by a tendency for the molten solder to spread along the trace as the result of capillary action and the weight of the component. Thus, it has heretofore been necessary to apply a dam of a material that is not wettable by the molten solder, which is referred to as a solder stop, to confine the molten solder to the terminal pad. By limiting the wettable area of the pad, each bump forms a microdroplet during reflow. The several bumps cooperate to support the component spaced apart from the board. Upon cooling, the microdroplets resolidify to form discrete interconnections. In contrast, in the absence of the solder stop, the molten alloy spreads onto the runner, leading to collapse of the component against the board and failing to produce the desired discrete interconnections. Nevertheless, the solder stop requires additional steps to apply and pattern, which adds to the cost of the package.
Another difficulty arises due to oxidation of the copper trace surface. The molten solder refuses to wet the oxidized surface, and thus does not form the desired bond. In the manufacture of packages that include discrete components, such as individual resistors and capacitors connected to a printed circuit board by wire leads, it is known to electroplate a solder film onto the copper trace to prevent oxidation and thereby provide a plated trace conducive to soldering. However, because the solder plate is applied continuously over the trace, it is not limited to the terminal pad, and thus has not heretofore been suited for bonding bump interconnections.